1. Field of the Invention
The present invention relates to improvements in the separator interposed between the positive and negative electrodes of an alkaline manganese dry battery. More particularly, the present invention relates to a dry battery separator conducive to an improved battery performance such as an increased discharge life, prevention of internal short-circuits, and so on.
2. Description of the Prior Art
The separator which separates the positive and negative electrode active materials from each other must have the following qualities.
(1) The positive and negative electrode active substance particles should not migrate to the electrodes of opposite sign, that is to say there should not be an internal short-circuit of the cell. PA1 (2) The separator can retain a sufficient amount of electrolyte such as KOH. PA1 (3) The separator has good processability, e.g. form-adaptability, mechanical strength and elongation, etc. PA1 (4) The separator as such is resistant to chemicals, not attacked by the active materials and electrolyte.
A variety of improvements have been proposed to provide separators possessing such properties in one. For example, Japanese Patent Publication No. 53-11059 proposes the combined use of a rayon fiber having a fineness of 1.5 deniers and a synthetic fiber having a fineness of 1.0 denier and states that the abovementioned requirements are fulfilled when these fibers are used in a given ratio. As is apparent from this patent literature, the fiber materials constituting the separator available today are fairly large in diameter, e.g. 1.0 to 1.5 dr or more. Separators composed of such relatively large fibers were fully satisfactory in chemical resistance and electrolyte-imbibition characteristics but as electrical appliances are made more or more compact, the sheet products made up of large-diameter fibers cannot display the expected functions. Thus, as the typical separator available today is made of large-denier fibers, it contains air spaces of the order of several .mu.m so that particles of the positive electrode active material manganese dioxide and graphite and the negative electrode active material zinc may migrate to the electrodes of opposite sign through the air spaces to cause an internal short-circuit. To prevent this short-circuit, it is common practice to use 2 or 5 sheets together as a laminate separator. However, while such a laminar separator serves to prevent the internal short-circuit, the overall thickness of the separator becomes so large that the separator occupies a large proportion of the cell space to decrease the available space for active materials and, hence, leads to a reduced battery life. Furthermore, such a laminated separator cannot be incorporated in UM-4 and smaller dry cells and thin button-type cells for structural reasons. Thus, battery separators made of large denier fibers are unsuitable for present-day applications.